Module-based oxy-fuel boiler

ABSTRACT

A boiler system for producing steam from water includes a plurality of serially arranged oxy fuel boilers. Each boiler has an inlet in flow communication with a plurality of tubes. The tubes of each boiler form at least one water wall. Each of the boilers is configured to substantially prevent the introduction of air. Each boiler includes an oxy fuel combustion system including an oxygen supply for supplying oxygen having a purity of greater than 21 percent, a carbon based fuel supply for supplying a carbon based fuel and at least one oxy-fuel burner system for feeding the oxygen and the carbon based fuel into its respective boiler in a near stoichiometric proportion. The oxy fuel system is configured to limit an excess of either the oxygen or the carbon based fuel to a predetermined tolerance. The boiler tubes of each boiler are configured for direct, radiant energy exposure for energy transfer. Each of the boilers is independent of each of the other boilers.

BACKGROUND OF THE INVENTION

The present invention pertains to an oxygen fueled boiler. Moreparticularly, the present invention pertains to a module-based oxy-fuelboiler having a flexible design.

The advantages of oxy-fuel combustion systems are well recognized. Forexample, Gross, U.S. Pat. Nos. 6,436,337 and 6,596,220, provide thatsome of the advantages of oxy-fuel combustion systems are reducedenvironmental pollution (reduced NOx generation), high efficiency, highflame temperatures and smaller overall physical plant design. The Grosspatents, which are commonly owned with the present application areincorporated herein by reference.

In order to extract the energy from the fuel, boilers typically providesome manner in which energy is input to a fluid (through combustion ofthe fuel) generally to change the state of the fluid. Energy is thenextracted from the fluid typically in the form of mechanical movement(or kinetic energy). Most boilers use water as the working fluid toextract energy from the fuel. Water is passed through tubes that formone or more “walls” or bundles within the boiler.

Typically, boiler tube walls are designed to transfer energy (in theform of heat) through the tube wall into the water in several loops andpasses of the walls. As the water passes through the tubes, the water isheated, under pressure and brought to a high level of energy (and phasechange) through super-heat, re-heat and/or super critical stages. Otherstages, such as an economizer unit may also be used through which wateris passed in furnace wall sections prior to super-heat passes. The wateris further heated by convective heat transfer from the heated gasesflowing past the tube bundles (e.g., in the economizer).

Each of the stages or regions of the boiler is designed to operate basedupon a certain type of heat transfer mechanism or phenomena. Forexample, the lower furnace walls are designed for radiant heat transferwhereas the upper bundles, super-heat, re-heat and economizer sectionsare designed to function on a convective heat transfer principle. Itwill be recognized by those skilled in the art that the heat transfermechanisms are not exclusive of one another as water is heated in theboiler.

Although such boiler configurations continue to serve their applicationsand purposes well, they do not necessarily take full advantage of thehigh flame temperatures and low exhaust gas volumes of oxy-fuelcombustion systems. Accordingly, there is a need for a boiler that usesan oxy-fuel combustion system to reduce environmental pollution.Desirably, such a boiler design provides high efficiency (vis-á-vis ahigh ratio of heat transferred to the working fluid to the heatavailable from the combustion products) and makes use of high flametemperatures. Most desirably, such a boiler configuration can provide asmaller overall physical plant design.

BRIEF SUMMARY OF THE INVENTION

A module based boiler system uses a plurality of independent, seriallyconfigured oxy fuel boilers for producing steam from water. The boilersare configured to carry out a different energy transfer function fromone another. A first or main boiler has a feedwater inlet in flowcommunication with a plurality of tubes for carrying the water. Theboilers are configured to substantially prevent the introduction of air.

The tubes of the main boiler form at least one water wall. Each boilerincludes an oxygen supply for supplying oxygen having a purity ofgreater than 21 percent and preferably at least about 85 percent, acarbon based fuel supply for supplying a carbon based fuel and at leastone oxy-fuel burner system. The burner system feeds the oxygen and fuelinto the boiler in a near stoichiometric proportion to limit an excessof either the oxygen or the carbon based fuel to a predeterminedtolerance. The tubes of each boiler are configured for direct, radiantenergy exposure for energy transfer from the flame to the water walltubes. In deference to traditional nomenclature, reference to waterwalls is intended to include all boiler tubes in a radiant zone eventhough the tubes may carry steam.

In one embodiment of the boiler system, the second boiler is a superheatboiler and steam produced by the first boiler is fed directly to thesuperheat boiler. Steam exits the superheat boiler and flows to a mainsteam turbine. Alternately, the system can include a reheat boiler(which takes feed from the high pressure steam turbine exhaust), reheatsthe steam in an oxy fuel boiler similar to the main boiler, and feeds areheat steam turbine. The energy transfer or heating function of each ofthe boilers is different from each of the other boilers. That is, in themain boiler, water is heated from a relatively low energy (enthalpy)value to saturated steam. In the superheat boiler (if used), the steamis further heated to superheated conditions. Then, in the reheater, theexhaust steam from the high pressure turbine is reheated for feeding toa reheat steam turbine.

The boiler system can include a condenser configured such that steamexhausts from the high pressure steam turbine to one or more reheatsteam turbines to optionally one or more low pressure turbines and on tothe condenser. A preferred boiler system includes an economizer. Theeconomizer has a gas side that receives combustion products (“exhaustgases” or “flue gases”) from the boilers and a feedwater side such thatthe combustion products preheat the boiler feedwater prior tointroducing the feedwater to the main boiler. Following exhaust from theeconomizer, the exhaust gases can be used to preheat the oxidizing agentfor the oxy-fuel combustion system, generally tying in to the exhaustgases system prior to any downstream exhaust gas processing treatmentthat may be desired. Increased power can be achieved by parallelgroupings of modular boiler systems.

The oxy-fuel burners can be configured for many different types of fuel,such as natural gas, oil, coal and other solid fuels. When using a solidfuel, a portion of the exhaust gases (optionally mixed with oxygen) canbe used to carry the solid fuel into the boilers. The fuel feed gasescan be exhaust gases from downstream of the economizer.

These and other features and advantages of the present invention will beapparent from the following detailed description, in conjunction withthe appended claims.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The benefits and advantages of the present invention will become morereadily apparent to those of ordinary skill in the relevant art afterreviewing the following detailed description and accompanying drawings,wherein:

FIG. 1 is a schematic flow diagram of a single reheat/subcritical boilersystem having module based oxy fuel boilers embodying the principles ofthe present invention;

FIG. 2 is a schematic flow diagram of a non-reheat/subcritical boilersystem having module based oxy fuel boilers embodying the principles ofthe present invention;

FIG. 3 is a schematic flow diagram of a single reheat/supercriticalboiler system having module based oxy fuel boilers embodying theprinciples of the present invention; and

FIG. 4 is a schematic flow diagram of a saturated steam boiler systemhaving a module based oxy fuel boiler embodying the principles of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

While the present invention is susceptible of embodiment in variousforms, there is shown in the drawings and will hereinafter be describeda presently preferred embodiment with the understanding that the presentdisclosure is to be considered an exemplification of the invention andis not intended to limit the invention to the specific embodimentillustrated.

It should be further understood that the title of this section of thisspecification, namely, “Detailed Description Of The Invention”, relatesto a requirement of the United States Patent Office, and does not imply,nor should be inferred to limit the subject matter disclosed herein.

An oxy-fuel combustion system uses essentially pure oxygen, incombination with a fuel source to produce heat, by flame production(i.e., combustion), in an efficient, environmentally non-adverse manner.Such a combustion system provides high efficiency (vis-á-vis a highratio of heat transferred to the working fluid to the heat availablefrom the combustion products) combustion and makes use of high flametemperatures. A preferred combustion system uses oxygen at a relativelyhigh purity (above about 21 percent and preferably at least about 85percent oxygen) and as such the overall volume of gas that passesthrough the boiler is commensurately lower. Using oxy-fuel, flametemperatures of greater than about 3000° F. and up to about 5000° F. inthe boiler are anticipated.

Moreover, one of the operational parameters of the present boiler systemis the use of an oxy-fuel combustion system in which relatively pureoxygen, rather than air, is used as the oxidizing agent. As used herein,oxidizing agent is intended to mean the gas that carries in the oxygenfor combustion. For example, when pure (100 percent) oxygen is suppliedto the system, the oxygen comprises 100 percent of the oxidizing agent,whereas when air is used as the oxidizing agent, oxygen comprises about21 percent of the oxidizing agent. Thus, the volume of oxidizing agentthat is needed is significantly less (because substantially only oxygenis used rather than air) than conventional boilers, which results in agas volume input (and thus throughput) to the boiler that is lower and agas flow rate through the boiler that is lower than conventionalboilers. One major advantage afforded by a lower flow rate and volume isthat the overall size of the physical plant system could be smaller thanconventional boiler systems and as such the capital cost of such aboiler system is anticipated to be commensurately lower.

One of the functional aspects or functional goals of the present boilersystem is to extract a maximum amount of energy (in the form of heattransfer from the combustion products/exhaust gases) from the combustionprocess. This, in conjunction with the lower flow rate, provides lessenergy loss at comparable exhaust gas stack temperatures.

Another aspect or functional goal of the present invention is to makethe maximum practicable use of the higher flame temperatures. As such,as will be described below, a considerably larger proportion of the heattransfer from the combustion products to the boiler tubes and hence tothe working fluid (water or steam) takes place by radiant heat transfer,rather than convective heat transfer.

A schematic illustration of one embodiment of a boiler system 10 isshown in FIG. 1. The illustrated system 10 is a reheat/subcritical unit.The system includes three separate and distinct boilers, namely boilerNo. 1 (main boiler 12); for producing steam from water, boiler No. 2(superheat boiler 14) for producing superheated steam, and boiler No. 3(reheat boiler 16). Oxygen and fuel are fed to each of the boilers byoxidizing agent and fuel supply systems 18, 20.

As illustrated schematically, and as will be discussed below, each ofthe boilers 12, 14, 16 includes its own independent oxy-fuel combustionsystem 22, 24, 26. In such an oxy-fuel combustion system, the waterwalls (tubes T see boiler 12 in FIG. 1) of each boiler 12-16 aresufficiently exposed to the flame that the major portion of heattransfer takes place by a radiant heat transfer mechanism rather than aconvective transfer mechanism. That is, the majority of the heattransfer occurs due to the direct flame exposure of the tubes, ratherthan the movement of heated exhaust gases over the tubes. This preferredradiant heat transfer mechanism is in sharp contrast to conventionalboilers that use large, long and complex exhaust gas flow paths (throughconvective passes, convective superheat passes, economizer sections andthe like), to maximize heat transfer trough convective mechanisms.

The present boiler system 10 further includes an economizer 28 thattransfers energy from boiler flue gases (preferably in all of theboilers) to the main boiler feed water (at the feedwater line 30) topreheat the feed water prior to introduction to the main boiler 12. In apresent system, oxygen is produced by separation from, for example, airin an oxygen generator 32. Those skilled in the art will recognize thevarious ways in which oxygen can be provided for feeding to the boilers12-16, for example, that oxygen can be supplied from sources such asstorage, water separation and the like, all of which are within thescope of the present invention. The fuel supply 20 can be any of varioustypes of fuels and various types of supplies. For example, the fuel canbe a gaseous fuel (e.g., natural gas), a liquid fuel such as fuel oil,diesel oil, or other organic or inorganic based liquid fuels, or a solidfuel such as coal, agricultural or livestock byproducts. All such oxygenproduction and supply configurations 18 as well as all such fuels andfuel supply arrangements 20 are within the scope and spirit of thepresent invention.

Returning now to FIG. 1, the boiler system 10 is shown as a supply foran electrical generator 34. To this end, the system includes aturbine/generator set 36 having the electrical generator 34, a highpressure or main steam turbine 38, an intermediate pressure steamturbine 40, a low pressure steam turbine 41 and a condenser 42.

The system 10 is configured such that feedwater enters the main boilerthrough feed water line 30 and is heated as it flows through the boiler12 water tubes T. In a typical boiler configuration, water enters theboiler 12 at a relatively low location in the boiler and rises throughthe tubes as it is heated. This serves to maintain the tubes in aflooded state and to maintain the fluid in the tubes at pressure.

The heated fluid is separated and saturated steam exits the main boiler12 through line 44 and enters the superheat boiler 14. Here, the steamis further heated to superheated conditions, again flowing through walltubes. The superheated steam exits the superheat boiler 14 through mainsteam line 46 and enters the high pressure (main steam) turbine 38. Thelower pressure steam exhausts from the high pressure main steam turbine38 and is returned to the reheat boiler 16 through the reheat steam line48. The steam exits the reheat boiler 16 through reheat steam flow line50 and enters the intermediate pressure turbine. The steam exhaustedfrom the intermediate turbine 40 flow through cross-over line 43 andenters the low pressure turbine 41.

The steam exhausts from the low pressure turbine 41 through the turbineexhaust line 52 and is fully condensed in the condenser 42 (generally ata low pressure—lower than atmospheric pressure—so that a maximum amountof energy is extracted by the turbine 40 from the steam) and is thenreturned (pumped) to the main boiler 12 through the economizer 28 which(as set forth above) preheats the water prior to introduction to theboiler 12.

As to the fuel circuit, as stated above, fuel and oxidizing agent arefed into each of the boilers 12, 14 and 16 independently. The flue gasesall exit their respective boilers through lines 13, 15 and 17,respectively, and enter the economizer 28 in which the gases preheat themain boiler feedwater. The flue gases exit the economizer 28 and can beused to preheat the oxidizing agent in oxidizing agent preheater 60. Theexhaust gases, after exiting the economizer 28 are routed to theoxidizing agent preheater 60 (through line 61) and are then returned(through line 63) for introduction to any necessary downstreamprocessing equipment indicated generally at 54, such as scrubbers,precipitators or the like. Additionally, in the event that it isdesired, a portion of the flue gas can be recirculated, generallyfollowing oxidizing agent preheat, (through flue gas recirculation lines56) to the boilers 12-16. The recirculation lines 56 can also be used asa vehicle (by diversion to fuel carrying lines 58) to carry fuel intothe boilers 12-16 to, for example, carry pulverized coal into theboilers.

As will be appreciated by those skilled in the art, because the flowrate and overall volume of gas entering the boiler (as substantiallypure oxygen) is less than conventional boilers, the flow rate and volumeof exhaust or flue gas is also commensurately less than conventionalboilers. As such the downstream processing equipment 54 can be smallerand less costly than conventional equipment of an equal sized (poweroutput) power plant.

A schematic illustration of a second embodiment of a boiler system 110is shown in FIG. 2. The illustrated boiler system 110 is anon-reheat/subcritical unit, and as such, the system includes twoseparate and distinct boilers, namely boiler No. 1 (main boiler 112) forproducing steam from water and boiler No. 2 (superheat boiler 114) forproducing superheated steam. There is no reheat boiler. This system 110is otherwise similar to the embodiment of the system 10 of FIG. 1 andincludes oxidizing agent and fuel supply systems 118, 120 (inindependent oxy-fuel combustion systems 122, 124) to independently feedeach of the boilers 112, 114. The boiler system 110 includes aneconomizer 128 that uses flue gas to preheat the feed water prior tointroduction to the main boiler 112. Exhaust gases after the economizer128 can be used to preheat the oxidizing agent in an oxidizing agentpreheater 160.

Here too, the boiler system 110 is configured with a turbine/generatorset 136 having an electrical generator 134, a high pressure (or mainsteam) turbine 138, an intermediate pressure turbine 140, a low pressureturbine 141 and a condenser 142.

Feed water enters the main boiler through feed water line 130 and isheated as it flows through the water tubes. The heated fluid isseparated and saturated steam exits the main boiler 112 through line 144and enters the superheat boiler 114 where the steam is heated to asuperheated condition. The superheated steam exits the superheat boiler114 through main steam line 146 and enters the high pressure turbine138. Unlike the previous embodiment, in this system 110, the steam thatexits the high pressure turbine 138 traverses through a cross-over line143 and enters the intermediate pressure turbine 140 (e.g., there is noreheater). The steam exits the intermediate pressure turbine 140 andtraverses through cross-over 148 and enters the low pressure turbine141. The low pressure steam is then exhausted from the low pressureturbine 141 through low pressure turbine to condenser line 152 and isthen returned (pumped) to the main boiler 112 through the economizer128.

As to the fuel circuit, as with the previous embodiment, fuel andoxidizing agent are fed into each of the boilers 112, 114 independently.The flue gases exit their respective boilers through lines 113 and 115,respectively, and enter the economizer 128 to preheat the main boilerfeedwater. The flue gases exit the economizer 128 and can be used topreheat the oxidizing agent in oxidizing agent preheater 160. Theexhaust gases, after exiting the economizer 128 are routed to theoxidizing agent preheater 160 (through line 161) and are then returned(through line 163) for introduction to any necessary downstreamprocessing equipment (as indicated at 154) following exit from theeconomizer 128. Flue gas can be recirculated 156 and/or used as avehicle to carry the fuel (e.g., pulverized coal) into the boilers 112,114.

Another embodiment of a boiler system 210 is illustrated in FIG. 3 whichshows a single reheat supercritical boiler unit. This system includestwo separate and distinct boilers, namely boiler No. 1 (supercriticalmain boiler 212) for producing supercritical steam from water and boilerNo. 2 (reheat boiler 216). Oxygen and fuel (in independent oxy-fuelcombustion systems 222, 226) are fed to each of the boilers 212, 216 byoxidizing agent and fuel supply systems 218, 220. The boiler system 210includes an economizer 228 that uses flue gas to preheat the feed waterprior to introduction to the main boiler 212.

Here too, the boiler system 210 is configured with a turbine/generatorset 236 having an electrical generator 234, a supercritical turbine 238,an intermediate pressure turbine 240, a low pressure turbine 241 and acondenser 242.

Feed water enters the main boiler 212 through feed water line 230 and isheated as it flows through the water tubes. The heated fluid exits thesupercritical boiler 212 through the supercritical steam line 246 andenters the supercritical turbine 238. The fluid (steam) exhausts fromthe supercritical turbine 238 enters the reheat boiler 216 throughreheat line 248 and then flows to the intermediate pressure turbine 240through reheat steam line 250. The steam exhausts from the intermediateturbine 240 through cross-over 243 into low pressure turbine 241. Thelow pressure steam exits the low pressure turbine 241 and is condensedin the condenser 242. The condensate is then returned (pumped) to thesupercritical boiler 212 through the economizer 228.

As to the fuel circuit, as with the previous embodiments, fuel andoxidizing agent are fed into each of the boilers 212, 216 independently.The flue gases exit their respective boilers through lines 213 and 217,respectively, and enter the economizer 228 to preheat the main boilerfeedwater. The flue gases exit the economizer 228 and can be used topreheat the oxidizing agent in oxidizing agent preheater 260. Theexhaust gases, after exiting the economizer 228 are routed to theoxidizing agent preheater 260 (through line 261) and are then returned(through line 263) for introduction to any necessary downstreamprocessing equipment 254 as necessary following exit from the economizer228. Flue gas can be recirculated 256 and/or used as a vehicle to carrythe fuel (e.g., pulverized coal) into the boilers.

Still another embodiment of a boiler system 310 is illustrated in FIG. 4which shows a saturated steam boiler unit. This system includes asaturated steam boiler 312 for producing saturated steam and an oxy-fuelcombustion system 322. The boiler system 310 can include an economizer328 that uses flue gas to preheat the feed water prior to introductionto the main boiler 312.

This boiler system 310 is configured to supply saturated steam to adesired (presently unspecified) downstream process 360. To this end, thesystem 310 is shown with a “steam requirement” (the downstream processrequiring steam) and a condenser 342, the need for which will dependupon the steam requirement 360.

Feed water enters the main boiler 312 through feed water line 330 and isheated as it flows through the water tubes. The heated fluid isseparated in, for example, a steam drum 313, into saturated steam andwater. The saturated steam exits the boiler 312 from the drum 313through the steam line 346 and flows to the steam requirement 360. Thefluid (steam) can then be condensed in the (optional) condenser 342,which would then be returned (pumped as feedwater) to the boiler 312through the economizer 328.

As to the fuel circuit, as with the previous embodiments, fuel andoxidizing agent are fed into the boiler 312 through an oxy-fuelcombustion system 322. The flue gases exit the boiler 312 through line313 and enter the economizer 328 to preheat the main boiler 312feedwater. The flue gases exit the economizer 328 and can be used topreheat the oxidizing agent in oxidizing agent preheater 370. Theexhaust gases, after exiting the economizer 328 are routed to theoxidizing agent preheater 370 (through line 371) and are then returned(through line 373) for introduction to any necessary downstreamprocessing equipment 354 as necessary following exit from the economizer328. Flue gas can be recirculated 356 and/or used as a vehicle to carrythe fuel (e.g., pulverized coal) into the boiler 312. Oxygen is suppliedby oxidizing agent supply 318 and fuel is supplied by fuel supply 320.

In each of the embodiments of the boiler system 10, 110, 210, 310, theboiler(s) are essentially stand alone units that are constructed tooperate so as to maximize heat transfer that occurs by way of a radiantheat transfer mechanism. As such, the boilers are relatively small (toensure effective exposure of the water walls/tubes T), or at leastsmaller than a comparable conventional boiler that relies on convectiveheat transfer. Those skilled in the art will recognize that althougheach of the boilers in each system (for example the main boiler 12,superheat boiler 14 and reheat boiler 16 of the single reheat boilersystem 10) is shown and described as a single boiler unit, it isanticipated that each of these boilers can be configured as multipleunits in series. Again, for example, the main boiler 12 could beconfigured as two or three smaller boilers in series. In addition,although each of the boilers is shown as having one oxy-fuel burner, itis anticipated that each boiler may have multiple burners, as need. Itwill be appreciated that the use of a single boiler or multiple boilersfor each of the heating stages and the use of a single burner ormultiple burners for each boiler will further enhance the ability tocontrol the heat input to the individual boilers to more efficientlycontrol the overall process and steam conditions.

As provided in the above-noted patents to Gross, energy is input to theboilers by the oxy-fuel combustion systems. Using such an arrangement,the principle mode of heat transfer to the furnace is radiant, with someconvective heat transfer. Because these burners (and the oxy-fuelsystems generally) produce high flame temperatures, the oxy fuelcombustion systems provide this efficient radiant heat transfer. Thegeometry of the boiler (e.g., direct flame exposure of the boiler tubes)further increases the heat transfer rate by maximizing the metal surfacearea over which heat transfer from the flame to the metal occurs.

Advantageously, the present boilers maximize the use of radiant heattransfer in combination with the use of oxy-fuel combustion which maypermit the boiler to be physically smaller than a conventional boiler ofan about equal size (power output). That is, because essentially pureoxygen (rather than air) is used as the oxidizing agent, the entirety ofthe oxidizing agent is available for combustion and the volume of gasinput to the boiler is about 21 percent of the volume of gas that wouldbe needed if air is used as the oxidizing agent to provide the oxygennecessary for combustion. Thus, the boiler could be considerably smallerbecause essentially pure oxygen rather than air is used.

In addition, the fuel/oxygen mixture (again, rather than a fuel/airmixture) results in higher flame temperatures in the boilers. Usingoxy-fuel, flame temperatures of about 5000° F. in the boiler can beachieved. This is higher, by about 1500° F. to 2000° F., thanconventional boilers. It has also been observed that using oxy-fuel, inconjunction with these higher flame temperatures, results in a highlyefficient process.

In present boiler systems using natural gas as fuel, the oxygen/naturalgas proportions are about 2.36:1. This ratio will vary depending uponthe purity of the oxygen supply and the nature of the fuel. For example,under ideal conditions of 100 percent pure oxygen, the ratio istheoretically calculated to be 2.056:1. However, in that the oxygensupply can have a percentage of non-oxygen constituents (generally up toabout 15 percent) and natural gas may not always be 100 percent pure,such a variation is expected. As such, those skilled in the art willappreciate and understand that the ratios may vary slightly, but thebasis for calculating the ratios, that is approximately stoichiometricproportions of fuel and oxygen, remain true.

This proportion of oxygen to fuel provides a number of advantages. Forexample, approximately stoichiometric proportions provide for completecombustion of the fuel, thus resulting in a substantially smaller volumeof NOx and other noxious off-gas emissions.

It is important to note that accurately controlling the ratio of oxygento fuel assures complete combustion of the fuel. This is in starkcontrast to conventional (for example, fossil fueled electric generationpower plants), that struggle with LOI (loss on ignition). Essentially,LOI equates to incomplete combustion of the fuel. The present boilersystems 10, 110, 210, 310, on the other hand, use substantially pureoxygen, in tightly controlled near stoichiometric proportion to the fuel(with boilers that are “tight”, that is, configured to essentiallyprevent the introduction of air), in an attempt to minimize and possiblyeliminate these losses. In addition, when using these burners (in anoxy-fuel system), the only theoretical NOx available is from fuel-bornenitrogen, rather than that which could otherwise result from combustionusing air. Thus, NOx, if not completely eliminated is reduced to aninsignificant amount compared to conventional combustion systems

Moreover, because radiant heat transfer is the desired heat transfermechanism, less reliance is made on convective (gas) passes within theboiler. This too permits a smaller, less complex boiler design. Thesedesign considerations allow the boilers to be configured as stand alone,modular units. That is, referring to FIG. 1, a stand alone main boiler12 can be grouped with a stand alone superheat boiler 14 which cangrouped with a stand alone reheat boiler 16. Likewise, referring to FIG.3, a stand alone supercritical main boiler 212 can be grouped with astand alone reheat boiler 216 as the core of the boiler system 210. Thisstand alone configuration gives control advantages over conventionalsystems where the temperature of the superheated steam is controlled byatemperation (desuperheat). The desuperheat process cools thesuperheated steam by the addition of water or steam (as vapor or spray)and drops the efficiency of the system and can be eliminated by usingseparate boilers for boiling and superheating. There are also advantagesduring turn down operation (operating at less capacity than designcapacity). Under turn down conditions the heat input into the boilingregion can be controlled independently of the heat input into thesuperheat region or reheat region and leads to more efficient operation.

A study of heat and mass balances around the various boilerconfigurations shows that the projected boiler efficiencies are quitehigh, and considerably higher than known boiler systems. For example, inthe first, reheat/subcritical unit, in the main boiler, the change inenthalpy of the water inlet to the steam outlet is about 1.95E9 BTU/hrwith a fuel input enthalpy of about 2.08E9 BTU/hr. In the superheatboiler, the change in enthalpy of the steam inlet to the steam outlet isabout 7.30E8 BTU/hr with a fuel input enthalpy of about 8.32E8 BTU/hr,and in the reheat boiler, the change in enthalpy of the water inlet tothe steam outlet is about 5.52E8 BTU/hr with a fuel input enthalpy ofabout 6.22E8 BTU/hr. These result in efficiencies in the main boiler,the superheat boiler and the reheat boiler of 93.8% (includingeconomizer gain), 87.8% and 88.7%, respectively.

Likewise, in the second, non-reheat, subcritical unit, in the mainboiler, the change in enthalpy of the water inlet to the steam outlet isabout 1.99E9 BTU/hr with a fuel input enthalpy of about 1.97E9 BTU/hr.In the superheat boiler, the change in enthalpy of the steam inlet tothe steam outlet is about 1.22E9 BTU/hr with a fuel input enthalpy ofabout 1.60E9 BTU/hr. These result in efficiencies in the main boiler andthe superheat boiler of 101.0% (including economizer gain) and 76.2%,respectively. It is important to note that the economizer is included inthe calculations for the main boiler (which takes exhaust from both theboiler and superheating boiler) and as such, credit is taken for theexhaust gas energy from the superheating boiler which allows theefficiency to appear to be greater than 100% (which it is not).

In the third, reheat-supercritical boiler, in the supercritical mainboiler, the change in enthalpy of the water inlet to the steam outlet isabout 2.37E9 BTU/hr with a fuel input enthalpy of about 2.72E9 BTU/hr.In the reheat boiler, the change in enthalpy of the steam inlet to thesteam outlet is about 6.23E8 BTU/hr with a fuel input enthalpy of about7.24E8 BTU/hr. These result in efficiencies in the supercritical mainboiler and the reheat boiler of 87.2% (including economizer gain) and86.0%, respectively.

In the last or the saturated steam boiler system, the change in enthalpyof the water inlet to the steam outlet is about 3.42E9 BTU/hr with afuel input enthalpy of about 3.73E9 BTU/hr. There is a blowdown loss ofabout 0.13E8 BTU/hr. This result in an efficiency in the main boiler of91.7%.

Table 1 below shows partial mass and energy balance components for thereheat/subcritical unit broken down by boilers. Table 2 shows partialmass and energy balance components for the non-reheat/subcritical unitbroken down by boilers, Table 3 shows partial mass and energy balancecomponents for the reheat-supercritical boiler unit broken down byboilers, and Table 4 shows partial mass and energy components for thesaturated steam boiler unit. It should be noted that the partial massand energy balance values in Table 3 for the reheat-supercritical boilerunit show first and second boiler sections, which have been addedtogether to determine the efficiency and to conform to the schematicillustration of FIG. 3. In each of the partial mass and energy balancevalue summaries in Tables 1-3, the specific and total enthalpy valuesare water inlet to the respective first combustion section before theeconomizer.

TABLE 1 Partial Mass and Energy Balance for Reheat/Subcritical BoilerSystem Specific Enthalpy Total Enthalpy Flow (lb/hr) (BTU/lb) (BTU/hr)Main Boiler Water Inlet 2,665,801   447.58 1,193,152,718 Steam Outlet2,639,407 1,190.70 3,142,753,707 Change in Enthalpy — — 1,949,600,989Fuel Inlet Enthalpy — — 2,078,881,200 Superheat Boiler Steam Inlet2,639,407 1,190.70 3,142,753,707 Steam Outlet 2,639,407 1,467.403,873,076,529 Change in Enthalpy — — 730,322,821 Fuel Inlet Enthalpy — —832,249,524 Reheat Boiler Steam Inlet 2,491,642 1,302.09 3,244,330,446Steam Outlet 2,491,692 1,523.59 3,796,230,559 Change in Enthalpy — —551,900,113 Fuel Inlet Enthalpy — — 622,050,305

TABLE 2 Partial Mass and Energy Balance for Non-Reheat/SubcriticalBoiler System Specific Enthalpy Total Enthalpy Flow (lb/hr) (BTU/lb)(BTU/hr) Main Boiler Water Inlet 3,338,027   486.17 1,622,853,948 SteamOutlet 3,304,978 1,093.39 3,613,619,311 Change in — — 1,990,765,363Enthalpy Fuel Inlet Enthalpy — — 1,969,222,441 Superheat Boiler SteamInlet 3,304,978 1,093,39 3,613,619,311 Steam Outlet 3,304,978 1,464.034,838,598,409 Change in Enthalpy — — 1,224,979,098 Fuel Inlet Enthalpy —— 1,602,594,525

TABLE 3 Partial Mass and Energy Balance for Reheat/Supercritical BoilerSystem Flow Specific Enthalpy Total Enthalpy (lb/hr) (BTU/lb) (BTU/hr)Supercritical Main Boiler (First section) Water Inlet 2,550,921   536.431,368,390,200 Steam Outlet 2,550,921 1,221.90 3,116,965,444 Change inEnthalpy — — 1,748,575,244 Fuel Inlet Enthalpy — — 1,995,760,950Supercritical Main Boiler (Second section) Steam Inlet 2,550,9211,221.90 3,116,965,444 Steam Outlet 2,550,921 1,466.18 3,740,107,987Change in Enthalpy — — 623,142,543 Fuel Inlet Enthalpy — — 724,457,506Reheat Boiler Steam Inlet 2,303,082 1,297.29 2,987,769,879 Steam Outlet2,303,082 1,543.11 3,553,906,020 Change in Enthalpy — — 566,136,141 FuelInlet Enthalpy — — 645,370,183

TABLE 4 Partial Mass and Energy Balance for Saturated Steam BoilerSystem Specific Enthalpy Total Enthalpy Main Boiler Flow (lb/hr)(BTU/lb) (BTU/hr) Water Inlet 3,401,777 170.78 580,969,557 Steam Outlet3,368,095 1,189.06 4,004,874,525 Change in Enthalpy — — 3,423,904,968Fuel Inlet Enthalpy — — 3,731,946,814

As set forth above, each of the boiler systems departs from conventionalprocesses in two principal areas. First, conventional combustionprocesses use air (as an oxidizing agent to supply oxygen), rather thanessentially pure oxygen, for combustion. The oxygen component of air(about 21 percent) is used in combustion, while the remaining components(essentially nitrogen) are heated in and exhausted from the furnace.Second, the present process uses oxygen and fuel in a nearstoichiometric proportion to one another (within a tolerance of about ±5percent). That is, only enough oxidizing agent is fed in proportion tothe fuel to assure complete combustion of the fuel within thepredetermined tolerance. And, this is carried out in multiple boilercomponents or modules configured as a coordinated system, each moduleheating in a respective, desired stage (e.g., main boiler, superheatregion, reheat region).

Many advantages and benefits are achieved using the present combustionsystem. It has been observed, as will be described below, that fuelconsumption, to produce an equivalent amount of power or heat isreduced. Significantly, this can provide for a tremendous reduction inthe amount of pollution that results. Again, in certain applications,the emission of NOx can be reduced to essentially zero.

In addition, it has been observed that because the throughput of gasesis considerably lower than conventional boilers the volume of dischargeof exhaust gases is commensurately lower. In fact, in that the input ofoxidizing agent (oxygen in the present system compared to air inconventional system) is about 21 percent of conventional systems, thedischarge is also about 21 percent of conventional systems (with solidfuels this may be, for example, 40 percent in that there is a quantityof motivating gas needed to move the solid fuel into the boiler). And,it is anticipated that the principle constituent of the discharge gaseswill be water (as vapor) which can be condensed or otherwise releasedand CO₂. It is also anticipated that the CO₂ is captured in concentratedform for use in other industrial and/or commercial applications and/orfor sequestration.

It has also been found that using a fuel/oxygen mixture (again, ratherthan a fuel/air mixture) results in higher flame temperatures asdiscussed above. Using oxy-fuel, flame temperatures of about 5000° F.can be achieved. This is higher, by about 1500° F. to 2000° F., thanother, known boilers. It has also been observed that using oxy-fuel, inconjunction with these higher flame temperatures, results in anextremely highly efficient process.

In the present disclosure, the words “a” or “an” are to be taken toinclude both the singular and the plural. Conversely, any reference toplural items shall, where appropriate, include the singular.

From the foregoing it will be observed that numerous modifications andvariations can be effectuated without departing from the true spirit andscope of the novel concepts of the present invention. It is to beunderstood that no limitation with respect to the specific embodimentsillustrated is intended or should be inferred. The disclosure isintended to cover by the appended claims all such modifications as fallwithin the scope of the claims.

1. A module based oxy-fuel boiler system for producing steam from water,comprising: a first boiler having a feedwater inlet in flowcommunication with a plurality of tubes for carrying the water, thetubes forming at least one water wall, the first boiler configured tosubstantially prevent the introduction of air; a first boiler oxygensupply for supplying oxygen having a purity of greater than 21 percent;a first boiler carbon based fuel supply for supplying a carbon basedfuel; at least one first boiler oxy-fuel burner system, the first boileroxy-fuel burner system feeding the oxygen and the carbon based fuel intothe first boiler in a near stoichiometric proportion to one another tolimit an excess of either the oxygen or the carbon based fuel to apredetermined tolerance, wherein the first boiler tubes are configuredfor direct, radiant energy exposure for energy transfer to the water toproduce steam; a second boiler having a plurality of tubes, the secondboiler being in series with the first boiler and configured to carry outa different energy transfer function than the first boiler, the tubes inthe second boiler forming at least one tube wall, the second boilerconfigured to substantially prevent the introduction of air; a secondboiler oxygen supply for supplying oxygen having a purity of greaterthen 21 percent; a second boiler carbon based fuel supply for supplyinga carbon based fuel; at least one second boiler oxy-fuel burner, thesecond boiler oxy-fuel burner feeding the oxygen and the carbon basedfuel into the second boiler in a near stoichiometric proportion to oneanother to limit an excess of either the oxygen or the carbon based fuelto a predetermined tolerance, wherein the second boiler tubes areconfigured for direct, radiant energy exposure for energy transfer toproduce steam, and wherein the first and second boilers are independentof and in series with one another.
 2. The module based oxy-fuel boilersystem in accordance with claim 1 wherein the first boiler oxygen supplysupplies oxygen having a purity of about 85 percent.
 3. The module basedoxy-fuel boiler system in accordance with claim 1 wherein the secondboiler oxygen supply supplies oxygen having a purity of about 85percent.
 4. The module based oxy-fuel boiler system in accordance withclaim 1 wherein the first boiler is a main boiler and the second boileris a superheat boiler and wherein steam produced by the first boiler isfed directly to the superheat boiler.
 5. The module based oxy-fuelboiler system in accordance with claim 4 including a steam turbine,wherein steam exiting the superheat boiler is fed to the steam turbine.6. The module based oxy-fuel boiler system in accordance with claim 5including a reheater boiler, wherein the reheater boiler has a pluralityof tubes, the reheater boiler being in series with the main boiler andthe superheat boiler and configured to carry out a different energytransfer function than the main boiler and the superheat boiler, thetubes in the reheat boiler forming at least one tube wall, the reheaterboiler configured to substantially prevent the introduction of air, thereheat boiler system including an oxygen supply for supplying oxygenhaving a purity of greater than 21 percent, a carbon based fuel supplyfor supplying a carbon based fuel and at least one reheat boileroxy-fuel burner, the oxy-fuel burner feeding the oxygen and the carbonbased fuel into the reheat boiler in a near stoichiometric proportion toone another to limit an excess of either the oxygen or the carbon basedfuel to a predetermined tolerance, wherein the reheat boiler tubes areconfigured for direct, radiant energy exposure for energy transfer tosuperheat the steam and wherein the reheat boiler is independent of themain boiler and the superheat boiler, the reheat boiler being fed froman exhaust of the steam turbine and configured to produce steam.
 7. Themodule based oxy-fuel boiler system in accordance with claim 6 whereinthe reheater boiler oxygen supply supplies oxygen having a purity ofabout 85 percent.
 8. The module based oxy-fuel boiler system inaccordance with claim 6 including an intermediate pressure turbine,wherein steam produced by the reheat boiler is fed to the intermediatepressure turbine.
 9. The module based oxy-fuel boiler system inaccordance with claim 8 including a low pressure turbine, wherein steamexhausted from the intermediate pressure turbine is fed to the lowpressure turbine and wherein steam exhausted from the low pressureturbine is fed to a condenser.
 10. The module based oxy-fuel boilersystem in accordance with claim 1 including an economizer having a gasside and a feedwater side, wherein exhaust gases from the first andsecond boiler flow into the economizer gas side and wherein feedwaterflows through the economizer and into the feedwater inlet.
 11. Themodule based oxy-fuel boiler system in accordance with claim 10 whereinthe first and second boilers are solid fuel boilers and wherein aportion of the exhaust gases is used to carry solid fuel into at leastone of the boilers.
 12. The module based oxy-fuel boiler system inaccordance with claim 11 wherein a portion of the exhaust gases is usedto carry solid fuel into the first and seconds boilers.
 13. The modulebased oxy-fuel boiler system in accordance with claim 11 wherein theportion of the exhaust gases that is used to carry solid fuel into atleast one of the boilers exhausts from an exhaust gas flow pathdownstream of the economizer.
 14. The module based oxy-fuel boilersystem in accordance with claim 10 wherein exhaust gases exhausting fromthe economizer gas side preheat the oxygen supply for the first andsecond boiler oxygen supplies.
 15. The module based oxy-fuel boilersystem in accordance with claim 1 wherein the first boiler is a mainboiler and the second boiler is a reheat boiler and including a mainsteam turbine and an intermediate pressure turbine, wherein steamexiting the main boiler is fed to the main steam turbine, steamexhausted from the main turbine is fed to the reheat boiler and steamexiting the reheat boiler is fed to the intermediate pressure turbine.16. The module based oxy-fuel boiler system in accordance with claim 15including a low pressure turbine, wherein steam exhausts from theintermediate pressure turbine to the low pressure turbine.
 17. Themodule based oxy-fuel boiler system in accordance with claim 16including a condenser and wherein steam exhausting from the low pressureturbine exhausts to the condenser.
 18. The module based oxy-fuel boilersystem in accordance with claim 15 including an economizer having a gasside and a feedwater side, wherein exhaust gases from the main andreheat boiler exhaust through the economizer and wherein feedwater fromthe condenser flow through the economizer and into the feedwater inlet.19. The module based oxy-fuel boiler system in accordance with claim 18wherein the main and reheat boilers are solid fuel boilers and wherein aportion of the exhaust gases is used to carry solid fuel into at leastone of the boilers.
 20. The module based oxy-fuel boiler system inaccordance with claim 19 wherein a portion of the exhaust gases is usedto carry solid fuel into the main and reheat boilers.
 21. The modulebased oxy-fuel boiler system in accordance with claim 20 wherein theportion of the exhaust gases that is used to carry solid fuel into atleast one of the boilers exhausts from an exhaust gas flow pathdownstream of the economizer.
 22. The module based oxy-fuel boilersystem in accordance with claim 18 wherein exhaust gases exhausting fromthe economizer gas side preheat the oxygen for the main and reheatboiler oxygen supplies.
 23. A boiler system for producing steam fromwater, comprising a plurality of serially arranged boilers, each boilerhaving a inlet in flow communication with a plurality of tubes forcarrying the water, the tubes forming at least one tube wall, each ofthe boilers configured to substantially prevent the introduction of air,each of the boilers including an oxygen supply for supplying oxygenhaving a purity of greater than 21 percent, a carbon based fuel supplyfor supplying a carbon based fuel and at least one oxy-fuel burnersystem for feeding the oxygen and the carbon based fuel into itsrespective boiler in a near stoichiometric proportion to limit an excessof either the oxygen or the carbon based fuel to a predeterminedtolerance, wherein the boiler tubes of each boiler are configured fordirect, radiant energy exposure for energy transfer, wherein each of theboilers is independent of each of the other boilers.
 24. The boilersystem in accordance with claim 23 wherein the boiler oxygen supplieseach supply oxygen having a purity of about 85 percent.
 25. The boilersystem in accordance with claim 23 including multiple pluralities ofserially arranged boilers, each of the multiples being in parallel withone another.
 26. The boiler system in accordance with claim 25 whereineach of the multiples is similar to each of the others of the multiples.27. A method for superheating steam from another boiler in a superheatboiler, said superheat boiler including: a steam inlet for receivingsaid steam; a plurality of boiler tubes in flow communication with saidsteam inlet forming at least one tube wall for receiving said steam,wherein said superheat boiler configured to substantially prevent theintroduction of air; an oxygen supply for supplying oxygen having apurity of greater than 21 percent; a carbon based fuel supply forsupplying a carbon based fuel; at least one oxy-fuel burner forreceiving said carbon based fuel and said oxygen having a purity ofgreater than 21 percent; a control system for feeding the oxygen and thecarbon based fuel into the second boiler oxy-fuel burner in a nearstoichiometric proportion to one another to limit an excess of eitherthe oxygen or the carbon based fuel to a predetermined tolerance,wherein the second boiler tubes are configured for direct, radiantenergy exposure for energy transfer to produce steam; a, the methodcomprising the steps of: (a) feeding steam from said other boiler to asuperheat boiler: and (b) superheating steam from said other boiler tocreate superheated steam.
 28. The method as recited in claim 27, furtherincluding the step of: (c) feeding said superheated steam to a highpressure turbine.
 29. The method as recited in claim 28, furtherincluding the step of: (d) feeding steam exhausted from said highpressure turbine to a reheat boiler, said reheat boiler being similar tosaid superheat boiler.
 30. The method as recited in claim 29, furtherincluding the step of: (e) feeding steam from said reheat boiler to anintermediate pressure turbine.
 31. The method as recited in claim 30,further including the step of: (f) feeding steam from said intermediatepressure turbine to a low pressure turbine.
 32. The method as recited inclaim 30, further including the step of: (g) condensing the steam fromthe low pressure turbine.
 33. The method as recited in claim 27, whereinsaid other boiler is similar to said reheat boiler.
 34. A module basedoxy-fuel boiler system comprising: a first boiler configured as a mainboiler and a second boiler configured as a reheat boiler, said firstboiler configured to substantially prevent the introduction of air andincluding: a plurality of first boiler tubes for carrying water, thetubes forming at least one water wall, a feedwater inlet line in flowcommunication with said plurality of first boiler tubes, said feedwaterinlet line configured to be connected to an external source of water, asteam outlet line in flow communication with said plurality of firstboiler tubes and configured to be connected to a first turbine, saidturbine having a steam outlet that is configured to be coupled to areheat line of said second boiler, a first boiler oxygen supply forsupplying oxygen having a purity of greater than 21 percent; a firstboiler carbon based fuel supply for supplying a carbon based fuel; atleast one first boiler oxy-fuel burner system coupled to said firstboiler oxygen supply and said first boiler carbon based supply, thefirst boiler oxy-fuel burner system configured to feed the oxygen andthe carbon based fuel into the first boiler in a near stoichiometricproportion to one another to limit an excess of either the oxygen or thecarbon-based fuel to a predetermined tolerance, a first boiler flue gasexit line for discharging exhaust gas generated as a result of thecombustion in said first boiler, wherein the first boiler tubes areconfigured for direct, radiant energy exposure for energy transfer tothe water to produce steam; a second boiler having a reheat inlet linein flow communication with the steam outlet of said turbine; a reheatsteam outlet line in flow communication with a second turbine, a secondboiler flue gas exit line for discharging exhaust gas generated as aresult of the combustion in said second boiler, wherein the secondboiler is configured to carry out a different energy transfer functionthan the first boiler and is configured to substantially prevent theintroduction of air; a second boiler supply for supplying oxygen havinga purity of greater then 21 percent; a second boiler carbon based fuelsupply for supplying a carbon based fuel; at least one second boileroxy-fuel burner system, the second boiler oxy-fuel burner systemconfigured to feed the oxygen and the carbon based fuel into the secondboiler in a near stoichiometric proportion to one another to limit anexcess of either the oxygen or the carbon based fuel to a predeterminedtolerance.
 35. The module based oxy-fuel boiler system in accordancewith claim 34, including an economizer having a gas side and a feedwaterside, wherein exhaust gases from the reheat boiler exhaust through aneconomizer.
 36. The module based oxy-fuel boiler system in accordancewith claim 34, wherein the main and reheat boilers are solid fuelboilers and wherein a portion of the exhaust gases is used to carrysolid fuel into at least one of the boilers.
 37. The module basedoxy-fuel boiler system in accordance with claim 35, wherein exhaustgases exhausting from the economizer gas side preheat the oxygen for themain and reheat boiler oxygen supplies.
 38. The module based oxy-fuelboiler system in accordance with claim 34, wherein the second boileroxygen supply supplies oxygen having a purity of about 85 percent.
 39. Amodule based oxy-fuel combustion system comprising: a main boiler havinga feedwater inlet in flow communication with a plurality of tubes T forcarrying water, the tubes forming at least one water wall, the firstboiler configured to substantially prevent the introduction of air; amain boiler oxygen supply for supplying oxygen having a purity ofgreater than 21 percent; a main boiler carbon based fuel supply forsupplying a carbon based fuel a main boiler oxy-fuel burner system, thefirst boiler oxy-fuel burner system feeding the oxygen and the carbonbased fuel into the main boiler in a near stoichiometric proportion toone another to limit an excess of either the oxygen or the carbon basedfuel to a predetermined tolerance, wherein the combustion produces aflame temperature of more than about 3000° F.; wherein the main boilertubes are configured for direct, radiant energy exposure for energytransfer to the water to produce steam; a secondary steam requirementconnected to said main boiler by way of a steam line for receivingsaturated steam from said main boiler and superheating said saturatedsteam.
 40. The module based oxy-fuel combustion system in accordancewith claim 38, wherein the main boiler oxygen supply supplies oxygenhaving a purity of about 85 percent.
 41. The module based oxy-fuelcombustion system in accordance with claim 38 wherein saturated steamproduced by the main boiler is fed directly to the secondary steamrequirement.